393 research outputs found
Stability of braneworlds with non-minimally coupled multi-scalar fields
Linear stability of braneworld models constructed with multi-scalar fields is
very different from that of single-scalar field models. It is well known that
both the tensor and scalar perturbation equations of the later can always be
written as a supersymmetric Schr\"{o}dinger equation, so it can be shown that
the perturbations are stable at linear level. However, in general it is not
true for multi-scalar field models and especially there is no effective method
to deal with the stability problem of the scalar perturbations for braneworld
models constructed with non-minimally coupled multi-scalar fields. In this
paper we present a method to investigate the stability of such braneworld
models. It is easy to find that the tensor perturbations are stable. For the
stability problem of the scalar perturbations, we present a systematic
covariant approach. The covariant quadratic order action and the corresponding
first-order perturbed equations are derived. By introducing the orthonormal
bases in field space and making the Kaluza-Klein decomposition, we show that
the Kaluza-Klein modes of the scalar perturbations satisfy a set of coupled
Schr\"{o}dinger-like equations, with which the stability of the scalar
perturbations and localization of the scalar zero modes can be analyzed
according to nodal theorem. The result depends on the explicit models. For
superpotential derived barane models, the scalar perturbations are stable, but
there exist normalizable scalar zero modes, which will result in unaccepted
fifth force on the brane. We also use this method to analyze the
braneworld model with an explicit solution and find that the scalar
perturbations are stable and the scalar zero modes can not be localized on the
brane, which ensure that there is no extra long-range force and the Newtonian
potential on the brane can be recovered.Comment: 13 pages, 3 figure
Gravitational resonances on -brane
In this paper, we investigate various -brane models and compare their
gravitational resonance structures with the corresponding general relativity
(GR)-branes. {Starting from some known GR-brane solutions}, we derive thick
-brane solutions such that the metric, scalar field, and scalar potential
coincide with those of the corresponding GR-branes. {We find that for branes
generated by a single or several canonical scalar fields, there is no obvious
distinction between the GR-branes and corresponding -branes in terms of
gravitational resonance structure.} Then we discuss the branes generated by
K-fields. In this case, there could exist huge differences between GR-branes
and -branes.Comment: 17 pages, 14 figures, published versio
Born-Infeld Black Holes in 4D Einstein-Gauss-Bonnet Gravity
A novel four-dimensional Einstein-Gauss-Bonnet gravity was formulated by D.
Glavan and C. Lin [Phys. Rev. Lett. 124, 081301 (2020)], which is intended to
bypass the Lovelock's theorem and to yield a non-trivial contribution to the
four-dimensional gravitational dynamics. However, the validity and consistency
of this theory has been called into question recently. We study a static and
spherically symmetric black hole charged by a Born-Infeld electric field in the
novel four-dimensional Einstein-Gauss-Bonnet gravity. It is found that the
black hole solution still suffers the singularity problem, since particles
incident from infinity can reach the singularity. It is also demonstrated that
the Born-Infeld charged black hole may be superior to the Maxwell charged black
hole to be a charged extension of the Schwarzschild-AdS-like black hole in this
new gravitational theory. Some basic thermodynamics of the black hole solution
is also analyzed. Besides, we regain the black hole solution in the regularized
four-dimensional Einstein-Gauss-Bonnet gravity proposed by H. L\"u and Y. Pang
[arXiv:2003.11552].Comment: 13 pages and 18 figures, published versio
Time-Dependent Scalar Fields in Modified Gravities in a Stationary Spacetime
Most no-hair theorems involve the assumption that the scalar field is
independent of time. Recently in [Phys. Rev. D90 (2014) 041501(R)] the
existence of time-dependent scalar hair outside a stationary black hole in
general relativity was ruled out. We generalize this work to modified gravities
and non-minimally coupled scalar field with an additional assumption that the
spacetime is axisymmetric. It is shown that in higher-order gravity such as
metric gravity the time-dependent scalar hair doesn't exist. While in
Palatini gravity and non-minimally coupled case the time-dependent
scalar hair may exist.Comment: 6 pages, no figure
Full linear perturbations and localization of gravity on brane
We study the thick brane world system constructed in the recently proposed
theories of gravity, with the Ricci scalar and the trace of
the energy-momentum tensor. We try to get the analytic background solutions and
discuss the full linear perturbations, especially the scalar perturbations. We
compare how the brane world model is modified with that of general relativity
coupled to a canonical scalar field. It is found that some more interesting
background solutions are allowed, and only the scalar perturbation mode is
modified. There is no tachyon state exists in this model and only the massless
tensor mode can be localized on the brane, which recovers the effective
four-dimensional gravity. These conclusions hold provided that two constraints
on the original formalism of the action are satisfied.Comment: v3: 8 pages, 2 figures, improved version with minor corrections,
accepted by EPJ
Cancer-cell microsurgery using nonlinear optical endomicroscopy
Near-infrared laser-based microsurgery is promising for noninvasive cancer treatment. To make it a safe technique, a therapeutic process should be controllable and energy efficient, which requires the cancer cells to be identifiable and observable. In this work, for the first time we use a miniaturized nonlinear optical endomicroscope to achieve microtreatment of cancer cells labeled with gold nanorods. Due to the high two-photon-excited photoluminescence of gold nanorods, HeLa cells inside a tissue phantom up to 250 μm deep can be imaged by the nonlinear optical endomicroscope. This facilitates microsurgery of selected cancer cells by inducing instant damage through the necrosis process, or by stopping cell proliferation through the apoptosis process. The results indicate that a combination of nonlinear endomicroscopy with gold nanoparticles is potentially viable for minimally invasive cancer treatment.<br /
A braneworld model in a massive gravity
A Randall-Sundrum-like braneworld scenario is constructed in a 5D extension
of the Lorentz-violating massive gravity. The gauge hierarchy problem is solved
in current model. The linear perturbations are calculated, and it is found that
the tensor and vector perturbations are robust and free from the ghost and
tachyonic instabilities, however, the scalar perturbation is a ghost filed.
After Kaluza-Klein reduction, all the tensor, vector and scalar modes are
massive and possess the mass splitting of order of TeV in their respective mass
spectra. The massive ground states of tensor and scalar modes propagate only
along the brane, however, the vector ground state is absent in the mass
spectrum. By introducing the Goldberger-Wise mechanism to stabilize the extra
dimension, the 4D effective theory on the brane includes a nearly massless
graviton plus three towers of very massive spin-2, spin-1 and ghost spin-0
particles.Comment: 24 pages, 2 figures, with some errors correcte
Detecting Extra Dimension By the Experiment of the Quantum Gravity Induced Entanglement of Masses
It is believed that gravity may be regarded as a quantum coherent mediator.
In this work we propose a plan using the Quantum Gravity Induced Entanglement
of Masses (QGEM) experiment to test the extra dimension. The experiment
involves two freely falling test masses passing though a Stern-Gerlach-like
device. We study the entanglement witness of them in the framework of
Randall-Sundrum II model (RS-II). It turns out that the system would reach
entangled more rapidly in the presence of extra dimension. In particular, this
is more significant for large radius of extra dimension
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